Cephalopods A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

, as active marine predators, possess sensory organs specialized for use in aquatic conditions.Budelmann BU. "Cephalopod sense organs, nerves and the brain: Adaptations for high performance and life style." Marine and Freshwater Behavior and Physiology. Vol 25, Issue 1-3, Page 13-33. They have a camera-type eye which consists of an iris, a circular lens, vitreous cavity (eye gel), pigment cells, and photoreceptor cells that translate light from the light-sensitive

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

into nerve signals which travel along the optic nerve to the brain. For the past 140 years, the camera-type cephalopod eye has been compared with the vertebrate eye as an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

, where both types of organisms have independently evolved the camera-eye trait and both share similar functionality. Contention exists on whether this is truly convergent evolution or parallel evolution. Unlike the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

camera eye, the cephalopods' form as invaginations of the body surface (rather than outgrowths of the brain), and consequently the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

lies over the top of the eye as opposed to being a structural part of the eye. Unlike the vertebrate eye, a cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The eye is approximately spherical, as is the

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

, which is fully internal. Cephalopods' eyes develop in such a way that they have retinal axons that pass over the back of the retina, so the optic nerve does not have to pass through the photoreceptor layer to exit the eye and do not have the natural, central, physiological blind spot of vertebrates. The crystallins used in the lens appear to have developed independently from vertebrate crystallins, suggesting a homoplasious origin of the lens. Most cephalopods possess complex extraocular muscle systems that allow for very fine control over the gross positioning of the eyes. Octopuses possess an autonomic response that maintains the orientation of their pupils such that they are always horizontal.

Polarized light

Several types of cephalopods, most notably squid and octopuses, and potentially cuttlefish, have eyes that can distinguish the orientation of polarized light. This sensitivity is due to the

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

organization of neighboring photoreceptors. (Cephalopods have receptor cells called

rhabdom The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part ...

s similar to those of other molluscs.) In contrast, the vertebrate eye is normally insensitive to polarization differences because the

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...

s in rods and cones are arrayed semi-randomly. And thus, the eye is equally sensitive to any orientation of the e-vector axis of the light. Because of their orthogonal organization, the opsins in cephalopod eyes have the highest light absorption when aligned properly with the light e-vector axis, allowing sensitivity to differences in polarization. The precise function of this ability has not been proven, but it is hypothesized to be for prey detection, navigation, and possibly communication among the color-changing cephalopods. File:Eye squid.jpg, Eye of '' Bathyteuthis'' sp. File:Octopusv cropped.JPG, Octopus ('' Octopus vulgaris'') eye File:Squid eye.jpg,

Squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

eye File:Cuttlefish eye closeup.JPG,

Cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...

eye File:Nautilus pompilius (head).jpg, Nautilus (''

Nautilus pompilius The chambered nautilus (''Nautilus pompilius''), also called the pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect logarithmic spiral, equiangular s ...

'') eye

Evolutionary debate

Disagreement on whether the evolution of the camera eye within cephalopods and within vertebrates is a parallel evolution or a

still exists, although is mostly resolved. The current standing is that of a convergent evolution for their analogous camera-type eye.

Parallel evolution

Those maintaining that it is a parallel evolution state that there is evidence that there was a common ancestor containing the genetic information for this eye development. This is evidenced by all bilaterian organisms containing the gene '' Pax6'' which expresses for eye development.

Convergent evolution

Those supporting a convergent evolution state that this common ancestor would have preceded both cephalopods and vertebrates by a significant margin. The common ancestor with the expression for camera-type eye would have existed approximately 270 million years before the evolution of camera-type eye in cephalopods and approximately 110 to 260 million years before the evolution of camera-type eye in vertebrates. Another source of evidence for this is the differences of expression due to independent variants of ''Pax6'' arising in both cephalopods and vertebrates. Cephalopods contain five variants of ''Pax6'' in their genomes which independently arose and are not shared by vertebrates, although they allow for a similar gene expression when compared to the ''Pax6'' of vertebrates.

Research and medical use

The main medical use emerging in this field is for research on eye development and ocular diseases. New research studies on ocular

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

are being performed using cephalopod eyes due to the evidence of their

with the analogous human eye. These studies replace the previous '' Drosophila'' studies for gene expression during eye development as the most accurate, although ''Drosophila'' studies remain the most common. The conclusion that they are analogous lends credibility to their comparison for medical use in the first place, since the trait in both would have been shaped through

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

by similar pressures in similar environments; meaning there would be similar expression of ocular disease in both organisms’ eyes. An advantage of cephalopod eye experimentation is that cephalopods can regenerate their eyes due to their ability to re-enable their developmental processes, which allows studies of the same cephalopod to continue past one trial sample when studying the effects of disease. This also permits for a more complex study concerning how regeneration may be conserved in cephalopod genomes and if it may be somewhat conserved in the human genome alongside the genes expressing for the camera eye.

References